Wood mills in North America and other parts of the world perform processing of timber to produce, among other products, dimensioned lumber for use in construction. The basic process flow begins with the saw mill, where a log is sawed into rough dimensioned lumber. The lumber is then cut to specific lengths and sorted into separate bins depending upon the species and rough dimensions. The sorted lumber is then bundled into packages and moved to a staging area to be readied for the kiln-drying process.
The kiln-drying process starts with stacking multiple packages atop one another in rows and columns called stacks. The combined stacks encompass a “charge” of lumber. Each charge of lumber is dried inside a kiln over a period of time until the charge is considered to be at the appropriate level of average moisture content. At that time, the kiln is turned off and the charge of lumber is removed. The packages are moved to a staging area where they are planed down to a final dimension and graded to determine value. Value depends on the general shape of the lumber and the moisture content.
Ideally, every board will be at the ideal moisture content, but such optimization is not practical due to the large volume of boards and imperfections in the drying process. Thus, the mills strike a balance between over-drying and under-drying, as either can cause lumber to lose value. For example, over-drying increases the amount of crook, warping and checking, which all degrade the value of lumber. On the other hand, under-drying can produce mold, also devaluing the lumber. Current standards allow a small percentage, about 5%, of their lumber to be above a critical threshold value of moisture content (usually around 19%).
Currently, if a mill has under-dried lumber, they have several options. First they can measure moisture content of each board and drop out the wet ones for re-drying. Second, they can sell the boards as “wet” lumber at a cost discount. Finally, they can allow the wet boards to pass through and be included with the dry lumber. However, selling wet boards increases the risk of the mill receiving a “wet claim”. Wet claims are known in the industry to be very costly to a mill.
Currently, mills employ different methods in determining when to shut down a kiln. Most kilns use an in-kiln moisture measurement system. This system is limited as it has only a small number of data points from which to estimate the average moisture content of a charge. Typically, such a system employs 4 to 12 sensor plates distributed throughout the kiln, with each sensor plate providing an average estimate of moisture content for a portion of a stack of lumber. One example of when to shut down the kiln is to average the sensor plate readings and compare the average to a predetermined threshold. When the threshold is surpassed, the kiln is shut down. Often, the in-kiln system moisture content determinations are not trusted and spot checks are performed with handheld meters in an attempt to confirm the in-kiln meter values. At other locations, in-kiln meters are not present, and operators rely on taking the spot checks with a handheld meter and averaging those values to determine the current average moisture content of the lumber.
There are several techniques for selecting the predetermined threshold for shutting down the kiln. First, customers can dictate the threshold value by requesting a specific average moisture content. Another method is to set the threshold value to a much lower value than is actually required to target a 100% certainty that the lumber will not produce a “wet” claim”. Another approach is to set the threshold based on a study performed in the distant past, and for months or years the same average threshold is employed, regardless of whether conditions have changed.
In any event, the prior art lacks a system that adequately maximizes the value of lumber.